In this bioarchaeological study of Classic Maya subsistence, we analyzed porotic hyperostosis, dental caries, and stable isotopes on skeletal remains recovered from 118 burials at Piedras Negras, Guatemala. The abundant porotic hyperostosis and dental caries at Piedras Negras, coupled with a high mean δ13C (mean -9.2 ± 1.0‰) provide evidence for high levels of maize consumption. The isotope data show slight differences among social status groups. Chronological changes in diet are more evident. The Early Classic (A.D. 350-625) skeletons are characterized by moderate heterogeneity in isotope ratios relative to skeletons of the Yaxche phase of the Late Classic (A.D. 625-750). In Yaxche phase skeletons, a prosperous time for Piedras Negras, δ13C and δ15N are homogenous, with an emphasis on dietary maize and terrestrial fauna. In Chacalhaaz times (A.D. 750-825) isotope signatures became much more diverse, with some individuals consuming less maize and/or more freshwater fish. We attribute these changes to significant perturbations in the system of food production and distribution at Piedras Negras in the late eighth century, a process that may have been linked to the disintegration of dynastic kingship at the site and its collapse.